If he did start in 2019, he hid it
super well. I never once suspected he was on coke, or any other drug, between 2019 and 2020.
Much hay has been made over him hiding his true personality back then, and that was certainly a thing, but I think it would not have been possible to hide hard drug usage that started that early. You've seen how much he's degraded physically and mentally over the past two years. I think if he started earlier, he would have degraded much faster.
I think
@Balldo's Gate is correct that the coke usage started at Hedonism II, circa late 2022.